Demand for electric power in Japan increases continuously year after year, but a fluctuation in the demand for electric power also tends to become remarkable according to heightening of the industrial structure and an improvement in national living standard. For example, when an amount of the daytime demand for the electric power summer is assumed to be 100, it is not more than 30 at dawn under the present conditions. Namely, the fluctuation in the demand for electric power has a great difference depending on time zones.
Since an electric power is supplied directly from generated power in a power station, if a fluctuation in the demand for electric power is large, the power station cannot help greatly fluctuating its output, this disturbs stable operation in the power station.
Particularly in recent years, a percentage of nuclear plants and new thermal power stations, which difficultly cope with output fluctuation in comparison with hydroelectric power or the like which can easily cope with output fluctuation, tends to increase. For this reason, necessity of facilities, which is capable of storing electric powers so as to cope with fluctuation in the demand for electric power while an output of a nuclear plant or the like is being maintained constant, is heightened. As such an electric power storing facility, a pumping power apparatus is currently used, but since installation of the pumping power apparatus requires a huge site, it is becoming difficult to secure such a site.
From the viewpoint of the above circumstances, various secondary batteries are being studied as a method of storing electric powers as energy which does not cause environmental pollution and has high versatility. Particularly, an attention is paid to a redox flow secondary battery which is constituted so that two kinds of redox agents are brought into contact via a diaphragm.
The redox flow secondary battery is such that an aqueous solution (electrolyte) of metallic ions whose valence changes is stored in a tank and this aqueous solution is supplied to a circulation type electrolytic cell having electrolytic cells so that charging and discharging take place.
As this redox flow battery, there typically suggest batteries using an iron-chromium hydrochloric acid solution as an electrolyte (for example, Japanese Patent Application Laid-Open No. 60-148068 (1985), Japanese Patent Application Laid-Open No. 63-76268(1988)), and batteries using vanadium sulfate solution as an electrolyte (for example, Japanese Patent Application Laid-Open No. 4-286871 (1992), Japanese Patent Application Laid-Open No. 6-188005(1994)).
However, as for the former batteries, preparation of an electrolyte is restricted from the viewpoint of mixing and solubility, and an output voltage is about 1V, namely, an energy density is low. Further, there arise problems that a charging state between the positive electrode solution and the negative electrode solution becomes imbalanced, that chlorine gas is possibly generated from the positive electrode at the time of charging, and the like. On the contrary, an attention is paid to the latter batteries because its output voltage is 1.4 V, namely, high, and thus this battery has high efficiency and high energy density.
Recently, there suggest some vanadium electrolyte producing methods, and for example, Japanese Patent Application Laid-Open No. 4-149965 (1992), Japanese Patent Application Laid-Open No. 5-290871 (1993), Japanese Patent Application Laid-Open No. 5-303973 (1993) and the like are known. They disclose methods of reacting a pentavalent vanadium compound with a reducing agent by means of electrolytic reduction or under existence of inorganic acid and collecting tetravalent and trivalent vanadium compound solutions so as to producing electrolytes.
In addition, the above-mentioned redox flow battery normally uses an electrolyte containing tetravalent vanadium as a positive electrode solution and an electrolyte containing trivalent vanadium as a negative electrode solution. This redox flow battery is such that the tetravalent vanadium in the positive electrode solution is changed into pentavalent vanadium and the trivalent vanadium in the negative electrode solution is changed into bivalent vanadium at the time of charging. At the time when the electrolytes in the positive electrode and negative electrode tanks become pentavalent and bivalent vanadium respectively, discharging takes place. However, charging and discharging in the electrolytes is balanced as long as a number of moles of the tetravalent vanadium oxidized with the positive electrode solution and a number of moles of the trivalent vanadium reduced by the negative electrode solution are balanced. For this reason, the electrolyte can be prepared without using a solution of only tetravalent vanadium or a solution of only trivalent vanadium. For example, it is known that a mixed vanadium solution, which contains tetravalent vanadium and the trivalent vanadium in the equal amount, is used as the positive electrode solution and the negative electrode solution, or a mixed solution of tetravalent vanadium and trivalent vanadium in 2:1 molar ratio is used as the positive electrode solution and a mixed liquid of tetravalent vanadium and trivalent vanadium in 1:2 molar ratio is used as the negative electrode solution.
Particularly, the mixed solution of the tetravalent and trivalent vanadium in 1:1 molar ratio does not require a balancing operation for the molar ratio and can be used commonly as the positive electrode solution and the negative electrode solution in the original state. For this reason, if the mixed vanadium compound which contains tetravalent and trivalent vanadium in 1:1 molar ratio can be produced easily, industrial utility value is high. As a method of producing a trivalent and tetravalent mixed vanadium compound, there suggest a method of producing a tetravalent and trivalent mixed electrolyte in such a manner that a vanadium compound is dissolved in a solvent under a condition of alkali or neutrality, vanadium ions are heated and polymerized under a condition of acidity so that polyvanadium oxide compound is separated, a part of the polyvanadium oxide compound is calcined in an atmosphere of inert gas or oxidation so that ammonium is removed, at least another part of the polyvanadium oxide compound is processed in an atmosphere of a reducing gas so that a trivalent vanadium compound is generated, pentavalent vanadium from the ammonia removing step is mixed and made to react with one part of the trivalent vanadium solution (Japanese Patent Application Laid-Open No. 08-148177 (1996)), a method of producing a trivalent and tetravalent vanadium electrolyte in such a manner that a reducing operation is performed on a compound containing pentavalent vanadium so that a vanadium compound with valence lower than pentavalent in which a heating peak of reoxidation is not more than 600° C. when this peak is measured by differential thermogravimetric analysis under airflow is generated, and an obtained reductant is mixed with the compound containing pentavalent vanadium so that the mixture is dissolved in a sulfate solution (Japanese Patent Application Laid-Open No. 11-67257(1999)), and the like.
However, since the conventional trivalent vanadium compound has unsatisfactory solubility with sulfuric acid, when the trivalent vanadium is tried to be dissolved with sulfuric acid and a redox flow battery electrolyte is prepared, a dissolving operation should be performed for several hours in a state that the electrolyte is heated to 100° C. For this reason, a special apparatus is required for preparing the electrolyte, and also a lot of trouble and time are required.
Therefore, it is an object of the present invention to provide a modified vanadium compound which can easily prepare a redox flow battery electrolyte, a producing method thereof, a redox flow electrolyte composite containing the modified vanadium compound and a redox flow battery electrolyte producing method.